1. Field of the Invention
The present invention relates to a method of making a laminate ceramic substrate having pads for connection with bumps of an electronic part, particularly of the kind in which all of or part of the pads have a domed shape.
2. Description of Related Art
A technique of making a laminate ceramic substrate by filling metallizing ink in through holes for vias (hereinafter will be referred to as "via holes") which are formed in ceramic green sheets at predetermined positions thereof and/or printing predetermined circuit patterns on the surface of the ceramic green sheets with metallizing ink, laminating a plurality of such green sheets in a predetermined order, and sintering the laminated green sheets, is known. With a recent demand for miniaturization and high density integration, a flip-chip technology is used as a technique for mounting and connecting an electronic part such as an IC (integrated circuit) chip to such a laminate ceramic substrate (hereinafter will be referred to simply as "substrate"). By the flip-chip technology, bumps are previously formed on the connecting ends of the electronic part while pads are formed on the substrate at corresponding positions to the bumps, and the bumps and pads are connected to each other.
There is also known such a modified kind of flip-chip technology wherein pads are formed on the mounting surface of a substrate on which an electronic part is mounted, nearly spherical or semispherical solder bumps are formed on an electronic part, and the pads and the solder bumps are subjected to heating while being held in contact with each other such that the solder bumps are melted to cause the pads and solder bumps to be connected to each other. This technology has an advantage in that it can dispense with the use of a metallic ball and therefore electronic devices can be produced at a lower cost as compared with the technology using metallic balls such that metallic balls are fixed by soldering to form metallic ball bumps which are then connected to pads by another soldering.
However, it was found that when the electronic part and substrate were connected in the above described manner, the solder bumps had a possibility of being reduced in the connecting strength. The connection of the electronic part to the substrate by means of the solder bumps can be tested for connecting strength by peeling off the substrate from the electronic part (this test is also referred to as "tensile breaking test"). When the connection is good, the soft solder (i.e., solder bump) is elongated in such a manner as to become narrower or thinner at a central portion thereof and then broken while allowing the solder having a nearly conical shape to be left behind on both of the electronic part side and the substrate side. By this test, it was found that some of the connected structures attained in the above described manner caused a fracture adjacent the substrate whose fractured surface had fine irregularities. The connected structured that causes such a fractured surface is easily broken in practical use and therefore low in reliability. This is because small voids (air holes) were included in the solder bumps after connection. It is believed that air and flux were enclosed in the space between the pad and the solder bump so that such air and cracked gas resulting from the heated flux were involved in the melted solder bump (i.e., solder) to cause such voids.
From further investigation, it was revealed that the surface of the pad in the form of a flat plate had small irregularities so that the junction between the pad and the solder bump was not attained substantially at one point but at a plurality of points or surfaces, and when the solder bump was heated, the solder bump started melting at the junction between the solder bump and the pad prior to the melting of the rest of the solder, and as such, the melted portion of the solder bump developed from the junction between the solder and the pad.
In contrast to this, when the pad is entirely or partly formed into a domed shape so as to contact the solder bump at one point, the solder of the solder bump starts melting at the point where it contacts the pad, when it melts, and wets and spreads over the pad. Due to this, the solder spreads out while expelling air and flux from the interface between the solder and the pad, so that air and cracked gas of flux do not become enclosed inside the solder to cause voids therewithin. Accordingly, a good result was obtained when the soldered assembly prepared in this way was subjected to the tensile breaking test. Further, since the electronic part and the substrate are connected so firmly, the connection therebetween can be highly reliable.
On the other hand, in view of existing circumstances in which the via holes are becoming smaller in diameter and becoming larger in number and with a view to providing a bonding pad having a good strength, Japanese patent provisional publication No. 3-112191 discloses a wired ceramic substrate with bonding pads each formed from a protruded end portion of a conductor layer or a via protruding from the via hole. Further, this document also discloses a method of forming such a bonding pad, as follows. That is, two kinds of conductive pastes having different coefficients of sintering contraction are prepared. One of the conductive paste having a larger difference in the coefficient of sintering contraction with respect to the green sheet (i.e., the conductive paste having a smaller coefficient of contraction) is filled under pressure into the via holes of the uppermost green sheet. On the other hand, the other of the conductive pastes having a smaller difference in coefficient of sintering contraction is filled into the via holes of other green sheets. Thereafter, those green sheets are placed one upon another and sintered at one time such that the conductive layers (i.e., vias) are caused to partially protrude from the via holes of the uppermost layer to form bonding pads.
However, in this technique, only the difference in the coefficient of contraction between the metallizing ink for via and the ceramic green sheet is used for the vias to protrude. Thus, when the difference of the coefficient of contraction is made larger to increase the amount of protrusion, radial cracks form in the ceramic layers around the vias into which the plating solutions intrude and remain, thus causing the drawback of reduced insulation resistance.
Further, a method usually used for filling of the ink under pressure is as follows. That is, as shown in FIG. 13A, on a rigid jig UH in the form of a flat plate such as a metal plate is placed a ceramic green sheet G having via holes H and on which is further placed a metal mask M having through holes MH at corresponding positions to the via holes H. Then, as shown in FIG. 13B, metallizing ink for via is forcedly filled by means of a squeegee or the like into the via holes H through the through holes MH of the metal mask M.
However, by this technique, frequently the end surface INa of the filled ink IN in contact with the jig UH in the form of a planar plate is not always formed flat or planar. For example, a portion of the end surface (e.g., a central portion) can be lower in level than the surrounding area, i.e., it can become depressed or irregularities as wrinkles or furrows can be caused in the end surface. While nothing definite is known about the cause for such depression or irregularities, it is believed that since the metallizing ink IN which is high in viscosity and therefore poor in filling ability is filled into the via holes H under pressure and the via holes H are closed at one end (i.e., the lower ends in the drawing) by the hard jig UH in the form of a flat plate, the filled ink IN has difficulty in forming a flat surface after the shape of the jig UH in the form of a flat plate, at the end of the via hole H. If the sheets G filled with ink IN having such end surfaces INa are laminated and sintered at the same time to form a substrate CS in which the vias MT are caused to protrude from the mounting surface Csa, due to the difference of the coefficient of sintering contraction, the protruded end MTa of each via MT, as shown in FIG. 13C, is not formed into such a domed shape that enables the protruded end to contact the solder bump at one point with as described above. Thus, the vias MT with such an end surface MTa may possibly cause voids in the solder bumps after connection of the vias MT to the solder bumps, thereby lowering their connecting strength and therefore the reliability of the connection.
In the meantime, a technique is also known in which after filling the metallizing ink for via, cover layers are printed onto the sheet in such a way as to cover the end surfaces of the filled masses of metallizing ink and then sintered to be used as pads.
Such a technique requires an additional printing process for cover layers and therefore increases the number of processes. Furthermore, due to the limits of improvement in the printing accuracy, there is difficulty in using such pads as those for fine pitch connection.